Many modem telephones have the capability of generating any selected one of a plurality of different ring tones. An example is the Avaya 4620 phone of Avaya Inc., which provides eight possible ring tones. Also, many cell phones support user-defined or downloaded ring tones, including music files. When telephones are being installed in an office, each telephone is assigned a default ring tone. For example, a script exists for the Asterisk open-source private-branch exchange (PBX) system that assigns ring tones to individual extension numbers or Internet protocol (IP) addresses.
The problem is that the phones in one area—that is, telephones that are physically perceived to be fairly close to each other—may be configured to generate the same default ring tone, and thus it can be very difficult to distinguish one ringing phone from another. The default ring-tone of a telephone can subsequently be changed to a different ring tone by a user of that telephone. Nevertheless, ensuring that ring tones of a large number of phones are sufficiently different from each other to be readily distinguishable is a challenge. Software such as the Heresy AI Composer from Wild Palm generates unique ring tones for cell phones, and claims to be theoretically capable of generating billions of compositions. Nevertheless, generating a unique ring tone for each phone is not necessarily a good solution, because it is difficult to ensure that, as the number of ring tones increases, the ring tones actually sound different from each other.